The present invention relates generally to teletext communication systems and, more particularly, to improvements in such systems enabling the efficient communication of selected special services information to limited groups of subscribers who, in general, are not part of the public teletext system.
Teletext is a generic term for a new and rapidly growing television based communication technique which uses the vertical blanking interval of a broadcast television signal for transmission of text and graphics information. Systems implementing this technique are presently operational in various countries, such as in England where teletext formated text and graphics material is transmitted in digitally encoded form on lines 17, 18, 330 and 331 of their 625 line system. Television receivers equipped with teletext decoders process the transmitted data for forming a display of the text and graphics on their viewing screens.
In the British teletext system, which is exemplary of most teletext systems presently in use, the broadcast teletext information is formated in a plurality of cyclically transmitted rows of data, twenty four rows comprising a page of information with each group of one hundred pages representing a magazine. Each row of information, which represents up to 40 characters of text or graphics data, is serially transmitted in digitally encoded form on one horizontal line of the vertical blanking interval so that the data transmission rate is two rows per field or four pages per second. In order to provide reasonable access time to the transmitted teletext information it is therefore customary to limit the cyclically transmitted data to 800 pages. Each teletext decoder includes a data acquisition circuit responsive to user commands for acquiring a selected page of the recycling teletext information and for storing the twenty four rows of digitally encoded data comprising the page. The stored encoded data is then applied to a display generator which decodes the stored information for synthesizing R, G, B video character signals which are coupled to and drive the cathode ray tube (CRT) of the television receiver for producing a display reflecting the stored data, the display comprising a page of twenty four rows of text or graphics with each row consisting of up to 40 characters. The first row, i.e. row 0, of each page is referred to as a page-header and includes an encoded magazine and page number as well as a row address. The remaining rows include only magazine and row address but not page number codes. Acquisition of a selected page of data is therefore achieved in the teletext decoder by initially matching the magazine and page number codes of a transmitted page-header row with user selected magazine and page number codes and then storing this matching page-header row together with the twenty three rows containing the corresponding magazine number. In this regard, it is to be appreciated that the data acquisition process cannot be affected until a match is achieved between the magazine and page number codes of a received page-header row and the magazine and page number codes selected by the user of the teletext decoder. The data acquisition time, i.e. the time between the selection by a viewer of a desired page and the presentation thereof on the screen of the receiver, is therefore dependent upon the relative position of the selected page in the cyclical transmission at the time the viewer enters the corresponding magazine and page number codes.
Teletext communication systems of the type generally described above serve a useful purpose in communicating a selected data base to the general public. Naturally, in order to appeal to the public at large and to justify the dedication of a portion of the broadcast television signal to this purpose, the selected data base must of necessity be oriented toward the mass consumer market. However, there exists numerous situations where it may be desired to provide communications of more limited applicability to a limited class of subscribers such as in the case of private newsletters, message services and the like. It would be desirable, from a cost effectiveness viewpoint, to employ existing teletext facilities and techniques to implement such services. But, in doing so, an implementation must be devised which does not significantly increase the access time by the general public to normal teletext communications and which makes the special services communications only available to the subscribers thereof and not to the owners of conventional teletext equipped receivers.
It is therefore a basic object of the invention to provide an improved teletext type communication system in which a selected general applicability data base may be communicated through a conventional teletext signal to each member of the general public having a teletext decoder equipped receiver while employing the same system to transmit special services information which may only be acquired by a limited group of subscribers.
It is another object of the invention to provide a teletext communication system of the foregoing type in which maximum use is made of existing teletext equipment and facilities.
A further object of the invention is to provide for the transmission of the special services information together with the normal teletext signal but without significantly affecting the access time thereto.